When performing optical analysis on a sample, it is desirable to account for background effects, for example the effects of the optical analysis cell and/or the presence of air or components thereof, which may affect the absorption characteristics in the frequency or frequencies of electromagnetic radiation used in the analysis.
In spectroscopic analysis, this can be achieved by first taking a background spectrum in the absence of any sample, followed by a spectrum in which the sample is present, and subtracting the former from the latter. However, the time delay between taking a spectrum of the background and the spectrum of the sample can often be significant. The delay allows the nature of the background to change (for example, a change in moisture composition of the surrounding air), which can reduce the quality of the background subtraction and also introduce noise into the analysis, which negatively impacts the accuracy of the measurements.
In order to resolve temporal effects in the background, dual-beam configurations can be used, in which two beams of electromagnetic radiation are simultaneously directed through separate paths, one containing the sample, and the other without sample. The absorption characteristics of the sample are calculated by subtracting the absorption/reflectance of the sample-free path from the sample-containing path. However, this type of experiment also suffers disadvantages, as the dual-beam equipment can be bulky, and background effects of any optical analysis cell containing the sample to be analysed are not taken into account, and have to be subtracted separately.
In GB 2,431,014, an optical sampling cell is described which has a concave cavity for holding a liquid sample, and in which the cell is made of material transparent to optical radiation. In one embodiment, a reflective surface is used to reflect radiation back through the sample. However, this arrangement does not avoid the need for carrying out a separate background subtraction.
U.S. Pat. No. 6,147,351 describes a method for analysing gas mixtures, in which the gas analysis cell comprises a partially reflective surface that reflects some radiation towards one detector, while allowing transmission of radiation to another detector. This is stated to enable the extent of collision broadening of peaks in a spectrum to be calculated.
DE 19 848 120 describes an arrangement in which a sample cell comprises two partially transmitting mirrors at either end, which causes some of the incident EMR to pass back and forward through the sample until being transmitted and detected. This is stated effectively to increase the sample pathlength, which improves sensitivity, while avoiding complex mirror and optical window arrangements. Additionally, U.S. Pat. No. 5,414,508, a cell is described having partially reflective surfaces on two sides of a sample channel, which improves detection of dilute species. However, the need for a separate background subtraction on the same apparatus in the absence of a sample is still required.
Therefore, there remains a need for improved apparatus and methods for obtaining an optical spectrum of a sample without the need to carry out separate background subtraction.